1. Technical Field
This invention relates to devices and methods for position determination and for surface measurement. In particular, the present invention relates to such devices and methods which use optical measurement methods, for example use a laser.
2. Background Information
Coordinate measuring devices are an example application field in which a position determination in a three-dimensional space is desired with an accuracy as high as possible. Coordinate measurement devices typically have a measurement head which for example may be provided with a tracer pin or other sensors. For determining positions on a surface of an object, the position of the measurement head and, if the tracer pin is mounted on the measurement head in a movable manner, the position of the tracer pin relative to the measurement head is determined. Conventionally, for determining a position of the measurement head linear scales are provided in the coordinate measurement device which feedback the position of the measurement head indirectly via movement paths of the individual axes. This, however, requires a solid mechanical construction to prevent a corruption of measurement results through a present looseness or through mechanical deformation.
Distances may be determined by a measurement of a path traveled by electromagnetic radiation, for example light. To achieve this, the electromagnetic radiation travels through a path between a reference position and the object once or a plurality of times, such that the distance may be derived from the length of the path traveled by the radiation.
The realization of devices and methods in which distances or object positions are determined in spaces having a length of several meters with an accuracy in the order of several micrometers or several ten micrometers is a technical challenge. This applies in particular if positions are to be determined with a high measurement rate.
Laser distance measurement devices enable determination of a distance of an object. In K. Minoshima and H. Matsumoto, “High-Accuracy Measurement of 240-m distance in an optical tunnel by use of a compact femtosecond laser”, Applied Optics, vol. 39, No. 30 pp. 5512-5517 (2000) a measurement of a distance using optical frequency combs is described. In this measurement the phase position of a signal component of the intensity of the laser beam frequency comb is evaluated to determine a distance traveled by the laser beam. The signal component is chosen such that it oscillates with a frequency which corresponds to a typically high multiple of the repetition rate of the laser beam. The measurement of a phase difference for such a signal component allows the determination of a position in a so-called unambiguousness area which is equal to the speed of light divided by the frequency of the signal component. To obtain an estimation of the distance which approximates the distance to be measured within the unambiguousness area for example DE 10 2008 045 386.2 of the present applicant proposes to evaluate sequentially different signal components of captured measurement signals, which oscillate with different frequencies. For this additional measurement an evaluation time is, however, needed.
Generally in optical methods for determining a position in a three-dimensional space based on travel time measurements or measurements of a phase difference, respectively, an object the position of which is to be determined is illuminated with a light beam, typically a laser beam, and the light reflected from the object, for example from a retroreflector mounted to the object, is detected. In this case it has to be ensured that the object, for example a measurement head of a coordinate measurement device as described above, is illuminated by the light beam in a complete measurement volume. Conventionally, the light beam is expanded via optical elements such that it illuminates the complete measurement volume. This expansion, however, leads to only a relatively small part of the laser beam being reflected from the object and therefore a signal intensity at a detector which is small compared with the incident laser power.
In other methods the reverse approach may be used, i.e. from the object, for example from a measurement head mounted to the object, one or more stationary reflectors are illuminated, and the reflected light is detected at the object. Also here it has to be ensured that independent from movements of the object in the space of interest the reflector or the reflectors is/are illuminated. Conventionally also in this case the light beam may be expanded with optical elements, which in turn leads to a low signal intensity.
Generally, it would be desirable to use the laser power more efficiently, to obtain a better signal-to-noise ratio at one or more detectors used and/or to be able to use lasers with reduced power.
Similar optical measurement methods, for example using laser light, may also be employed in surface measurements. Here for example a surface to be measured is illuminated with a laser, for example a short-pulse laser, and the light reflected from the surface is detected with one or more detectors. For a three-dimensional measurement at least three independent detectors are necessary, in case less dimensions are to be measured less detectors may be used correspondingly.
Also in such devices and methods it is desirable be able to detect as big a part as possible of the laser power reflected from the surface.